Two things come to mind. Did you do any work on the carbs since it ran good? Installing the jets in the wrong towers will cause what you are experiencing.
The other thing would be the high speed coil of the stator has given up. You might try the "White Wire Fix" just to see if it changes. Could also be the CDI giving up at high speeds. And probably a few more that others will post shortly.
You can check the voltages on the blue (high speed) and white (low speed) coils in the stator with a multimeter. Set the meter for AC VOLTAGE, and measure the voltage from each wire. You can even have it running for the blue wire. You should be seeing approx 90 volts on the white, and 100+ on the blue. Remember, measure at the wire coming from the stator, not the CDI. The White wire fix simply connects the blue wire going into the CDI with the white coming from the stator. This provides marginally high enough voltage for the high speed section of the CDI.